source: draft-ietf-httpbis-content-disp/00/draft-ietf-httpbis-content-disp.xml @ 1846

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<!DOCTYPE rfc [
  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
]>

<rfc xmlns:x="http://purl.org/net/xml2rfc/ext" xmlns:ed="http://greenbytes.de/2002/rfcedit" ipr="trust200902" docName="draft-ietf-httpbis-content-disp-00" category="std" x:maturity-level="proposed" xml:lang="en" updates="2616">
        <front>
  <title abbrev="Content-Disposition in HTTP">Use of the Content-Disposition&#160;Header&#160;Field
  in the Hypertext&#160;Transfer&#160;Protocol&#160;(HTTP)</title>
  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
    <organization abbrev="greenbytes">greenbytes GmbH</organization>
    <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <email>julian.reschke@greenbytes.de</email>       
      <uri>http://greenbytes.de/tech/webdav/</uri>      
    </address>
  </author>

  <date month="September" year="2010" day="3"/>
  <workgroup>HTTPbis Working Group</workgroup>
  
  <abstract>
    <t>
      HTTP/1.1 defines the Content-Disposition response header field,
      but points out that it is not part of the HTTP/1.1 Standard. 
      This specification takes over the definition and registration of
      Content-Disposition, as used in HTTP, and clarifies internationalization
      aspects.
    </t>
  </abstract>
  
  <note title="Editorial Note (To be removed by RFC Editor before publication)">
    <t>
      This specification is expected to replace the definition of Content-Disposition
      in the HTTP/1.1 specification, as currently revised by the IETF HTTPbis
      working group. See also <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/123"/>. 
    </t>
    <t>
      Discussion of this draft should take place on the HTTPBIS working group
      mailing list (ietf-http-wg@w3.org). The current issues list is
      at <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/query?component=content-disp"/>
      and related documents (including fancy diffs) can be found at
      <eref target="http://tools.ietf.org/wg/httpbis/"/>.
    </t>
    <!--<t>
      The changes in this draft are summarized in <xref target="changes.since.04"/>.
    </t>-->
  </note>
  </front>

  <middle>

<section title="Introduction" anchor="introduction">
<t>
  HTTP/1.1 defines the Content-Disposition response header field in <xref target="RFC2616" x:fmt="of" x:sec="19.5.1"/>,
  but points out that it is not part of the HTTP/1.1 Standard (<xref target="RFC2616" x:fmt="sec" x:sec="15.5"/>):
</t>
<x:blockquote cite="http://tools.ietf.org/html/rfc2616#section-15.5">
  <t>
    Content-Disposition is not part of the HTTP standard, but since it is
    widely implemented, we are documenting its use and risks for implementers.
  </t>
</x:blockquote>
<t>
  This specification takes over the definition and registration of
  Content-Disposition, as used in HTTP.
  Based on interoperability testing with existing User Agents, 
  it fully defines a profile of the
  features defined in the Multipurpose Internet Mail Extensions (MIME) variant (<xref target="RFC2183"/>) of the 
  header field, and also clarifies internationalization
  aspects.
</t>
</section>  

<section title="Notational Conventions">
<t>
  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
  are to be interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
  This specification uses the augmented BNF notation defined in
  <xref target="RFC2616" x:fmt="of" x:sec="2.1"/>, including its rules for
  linear whitespace (LWS).
</t>
</section>  

<section title="Header Field Definition" anchor="header.field.definition">
  <iref item="Headers" subitem="Content-Disposition" primary="true" x:for-anchor=""/>
  <iref item="Content-Disposition header" primary="true" x:for-anchor=""/>
<t>
  The Content-Disposition response header field is used to convey additional
  information about how to process the response payload, and also can be used
  to attach additional metadata, such as the filename.
</t>

  
<section title="Grammar">
<figure><artwork type="abnf2616">
  content-disposition = "Content-Disposition" ":"
                         disposition-type *( ";" disposition-parm )

  disposition-type    = "inline" | "attachment" | disp-ext-type
                      ; case-insensitive
  disp-ext-type       = token

  disposition-parm    = filename-parm | disp-ext-parm

  filename-parm       = "filename" "=" value
                      | "filename*" "=" ext-value
  
  disp-ext-parm       = token "=" value
                      | ext-token "=" ext-value
  ext-token           = &lt;the characters in token, followed by "*"&gt;
</artwork></figure>

<figure>
<preamble>Defined in <xref target="RFC2616"/>:</preamble>
<artwork type="abnf2616">
  token       = &lt;token, defined in <xref target="RFC2616" x:fmt="," x:sec="2.2"/>&gt;
  value       = &lt;value, defined in <xref target="RFC2616" x:fmt="," x:sec="3.6"/>&gt;
</artwork></figure>
<figure>
<preamble>Defined in <xref target="RFC5987"/>:</preamble>
<artwork type="abnf2616">
  ext-value   = &lt;ext-value, defined in <xref target="RFC5987" x:sec="3.2"/>&gt;
</artwork></figure>
</section>

<section title="Disposition Type" anchor="disposition.type">
<t>
  If the disposition type matches "attachment" (case-insensitively), this indicates that the user agent should not display the response,
  but directly enter a "save as..." dialog.
</t>
<t>
  On the other hand, if it matches "inline" (case-insensitively), this implies default processing.
</t>
<t>
  Other disposition types &SHOULD; be handled the same way as "attachment"
  (see also <xref target="RFC2183" x:fmt="," x:sec="2.8"/>).
</t>
</section>

<section title="Disposition Parameter: 'Filename'" anchor="disposition.parameter.filename">
<t>
  The parameters "filename" and "filename*", to be matched case-insensitively,
  provide information on how to construct a filename for storing the message
  payload.
</t>
<t>
  Depending on the disposition type, this information might be used right away
  (in the "save as..." interaction caused for the "attachment" disposition type),
  or later on (for instance, when the user decides to save the contents of the
  current page being displayed). 
</t>
<t>
  "filename" and "filename*" behave the same, except that "filename*" uses
  the encoding defined in <xref target="RFC5987"/>, allowing the use 
  of characters not present in the ISO-8859-1 character set (<xref target="ISO-8859-1"/>). When both "filename" and "filename*"
  are present, a recipient &SHOULD; pick "filename*" and ignore "filename" -
  this will make it possible to send the same header value to clients
  that do not support "filename*".
</t>

<t>
  It is essential that user agents treat the specified filename as advisory
  only, thus be very careful in extracting the desired information.
  In particular:
  <list style="symbols">
    <x:lt><t>
      When the value contains path separator characters, all but the last
      segment &SHOULD; be ignored. This prevents unintentional overwriting
      of well-known file system location (such as "/etc/passwd").
    </t></x:lt>
    <x:lt><t>
      Many platforms do not use Internet Media Types (<xref target="RFC2046"/>)
      to hold type information in the file system, but rely on filename
      extensions instead. Trusting the server-provided file extension could
      introduce a privilege escalation when later on the file is opened locally
      (consider ".exe"). Thus, recipients need to ensure that a file extension
      is used that is safe, optimally matching the media type of the received
      payload.
    </t></x:lt>
    <x:lt><t>
      Other aspects recipients need to be aware of are names that have a 
      special meaning in the filesystem or in shell commands, such as "." and "..",
      "~", "|", and also device names.
    </t></x:lt>
  </list>
</t>
</section>

<section title="Disposition Parameter: Extensions" anchor="disposition.parameter.extensionsS">
<t>
  To enable future extensions, unknown parameters &SHOULD; be ignored (see also <xref target="RFC2183" x:fmt="," x:sec="2.8"/>).
</t>
</section>

<section title="Extensibility" anchor="extensibility">
<t>
  Note that <xref target="RFC2183" x:fmt="of" x:sec="9"/> defines IANA registries both
  for disposition types and disposition parameters. This registry is
  shared by different protocols using Content-Disposition, such as MIME and HTTP.
  Therefore, not all registered values may make sense in the context of HTTP.
</t>
</section>

</section>  

<section title="Examples">

<figure>
<preamble>
Direct UA to show "save as" dialog, with a filename of "foo.html":  
</preamble>
<artwork type="example">
Content-Disposition: Attachment; filename=foo.html
</artwork></figure>
<figure>
<preamble>
Direct UA to behave as if the Content-Disposition header field wasn't present,
but to remember the filename "foo.html" for a subsequent save operation:
</preamble>
<artwork type="example">
Content-Disposition: INLINE; FILENAME= "foo.html"
</artwork></figure>
<figure>
<preamble>
Direct UA to show "save as" dialog, with a filename of "an example":
</preamble>
<artwork type="example">
Content-Disposition: Attachment; Filename*=UTF-8'<x:highlight>en</x:highlight>'an<x:highlight>%20</x:highlight>example
</artwork>
<postamble>Note that this example uses the extended encoding defined in
<xref target="RFC5987"/> to specify that the natural language of the filename
is English, and also to encode the space character which is not allowed in the
token production.
</postamble>
</figure>
<figure>
<preamble>
Direct UA to show "save as" dialog, with a filename containing the Unicode character  U+20AC (EURO SIGN):
</preamble>
<artwork type="example">
Content-Disposition: attachment; filename*= UTF-8''<x:highlight>%e2%82%ac</x:highlight>%20rates
</artwork>
<postamble>
  Here, the encoding defined in <xref target="RFC5987"/> is also used to encode the 
  non-ISO-8859-1 character.
</postamble>
</figure>


<figure>
<preamble>
Same as above, but adding the "filename" parameter for compatibility with
user agents not implementing RFC 5987:
</preamble>
<artwork type="example">
Content-Disposition: attachment; filename="EURO rates";
                                 filename*=utf-8''<x:highlight>%e2%82%ac</x:highlight>%20rates
</artwork>
<postamble>
  Note: as of August 2010, many user agents unfortunately did not properly handle
  unexpected parameters, and some that implement RFC 5987 did not pick
  the extended parameter when both were present. 
</postamble>
</figure>

</section>

<section title="Internationalization Considerations" anchor="i18n">
<t>
  The "filename*" parameter (<xref target="disposition.parameter.filename"/>),
  using the encoding defined in <xref target="RFC5987"/>, allows the 
  server to transmit characters outside the ISO-8859-1 character set,
  and also to optionally specify the language in use.
</t>
<t>
  Future parameters might also require internationalization, in which case
  the same encoding can be used.
</t>
</section>

<section title="Security Considerations" anchor="security.considerations">
<t>
  Using server-supplied information for constructing local filenames introduces
  many risks. These are summarized in <xref target="disposition.parameter.filename"/>.
</t>
<t>
  Furthermore, implementers also ought to be aware of the Security
  Considerations applying to HTTP (see <xref target="RFC2616" x:fmt="of" x:sec="15"/>), and also the parameter encoding defined in <xref target="RFC5987"/>
  (see <xref target="RFC5987" x:fmt="sec" x:rel="#security.considerations"/>). 
</t>
</section>  

<section title="IANA Considerations" anchor="iana.considerations">

<section title="Registry for Disposition Values and Parameter" anchor="registry">
<t>
  This specification does not introduce any changes to the registration
  procedures for disposition values and parameters that are defined in 
  <xref target="RFC2183" x:fmt="of" x:sec="9"/>.
</t>

</section>

<section title="Header Field Registration" anchor="header.field.registration"> 
<t>
  This document updates the definition of the Content-Disposition HTTP header field
  in the permanent HTTP header field registry (see <xref target="RFC3864"/>).
</t>
<t>
<list style="hanging">
  <t hangText="Header field name:">Content-Disposition</t>
  <t hangText="Applicable protocol:">http</t>
  <t hangText="Status:">standard</t>
  <t hangText="Author/Change controller:">IETF</t>
  <t hangText="Specification document:">this specification (<xref target="header.field.definition"/>)</t>
</list>
</t>
</section>

</section>  

<section title="Acknowledgements">
<t>
  Thanks to Rolf Eike Beer, Alfred Hoenes, and Roar Lauritzsen for 
  their valuable feedback.
</t>
</section>  

  </middle>
  <back>
  
<references title="Normative References">
  
  <reference anchor="RFC2119">
    <front>
      <title abbrev="RFC Key Words">Key words for use in RFCs to Indicate Requirement Levels</title>
      <author initials="S." surname="Bradner" fullname="Scott Bradner">
        <organization>Harvard University</organization>
        <address><email>sob@harvard.edu</email></address>
      </author>
      <date month="March" year="1997"/>
      <area>General</area>
      <keyword>keyword</keyword>
    </front>
    <seriesInfo name="BCP" value="14"/>
    <seriesInfo name="RFC" value="2119"/>
  </reference>

  <reference anchor="RFC2616">
    <front>
      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
      <author initials="R." surname="Fielding" fullname="R. Fielding">
        <organization>University of California, Irvine</organization>
        <address><email>fielding@ics.uci.edu</email></address>
      </author>
      <author initials="J." surname="Gettys" fullname="J. Gettys">
        <organization>W3C</organization>
        <address><email>jg@w3.org</email></address>
      </author>
      <author initials="J." surname="Mogul" fullname="J. Mogul">
        <organization>Compaq Computer Corporation</organization>
        <address><email>mogul@wrl.dec.com</email></address>
      </author>
      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
        <organization>MIT Laboratory for Computer Science</organization>
        <address><email>frystyk@w3.org</email></address>
      </author>
      <author initials="L." surname="Masinter" fullname="L. Masinter">
        <organization>Xerox Corporation</organization>
        <address><email>masinter@parc.xerox.com</email></address>
      </author>
      <author initials="P." surname="Leach" fullname="P. Leach">
        <organization>Microsoft Corporation</organization>
        <address><email>paulle@microsoft.com</email></address>
      </author>
      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
        <organization>W3C</organization>
        <address><email>timbl@w3.org</email></address>
      </author>
      <date month="June" year="1999"/>
    </front>
    <seriesInfo name="RFC" value="2616"/>
  </reference>

  <reference anchor="RFC5987">
        <front>
      <title abbrev="RFC2231 Encoding in HTTP">Applicability of RFC 2231
      Encoding to Hypertext Transfer Protocol (HTTP) Headers</title>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
        <organization abbrev="greenbytes">greenbytes GmbH</organization>
        <address>
          <postal>
            <street>Hafenweg 16</street>
            <city>Muenster</city><region>NW</region><code>48155</code>
            <country>Germany</country>
          </postal>
          <email>julian.reschke@greenbytes.de</email>   
          <uri>http://greenbytes.de/tech/webdav/</uri>  
        </address>
      </author>
      <date month="August" year="2010"/>
    </front>
    <seriesInfo name="RFC" value="5987"/>
  </reference>


  <reference anchor="ISO-8859-1">
    <front>
      <title>Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1</title>
      <author>
        <organization>International Organization for Standardization</organization>
      </author>
      <date year="1998"/>
    </front>
    <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
  </reference>

</references>
  
<references title="Informative References">

  <reference anchor="RFC2046">
    <front>
      <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
      <author initials="N." surname="Freed" fullname="Ned Freed">
        <organization>Innosoft International, Inc.</organization>
        <address><email>ned@innosoft.com</email></address>
      </author>
      <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
        <organization>First Virtual Holdings</organization>
        <address><email>nsb@nsb.fv.com</email></address>
      </author>
      <date month="November" year="1996"/>
    </front>
    <seriesInfo name="RFC" value="2046"/>
  </reference>

  <reference anchor="RFC2047">
    <front>
      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
      <author initials="K." surname="Moore" fullname="Keith Moore">
        <organization>University of Tennessee</organization>
        <address><email>moore@cs.utk.edu</email></address>
      </author>
      <date month="November" year="1996"/>
    </front>
    <seriesInfo name="RFC" value="2047"/>
  </reference>

  <reference anchor="RFC2183">
    <front>
      <title abbrev="Content-Disposition">Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field</title>
      <author initials="R." surname="Troost" fullname="Rens Troost">
        <organization>New Century Systems</organization>
        <address><email>rens@century.com</email></address>
      </author>
      <author initials="S." surname="Dorner" fullname="Steve Dorner">
        <organization>QUALCOMM Incorporated</organization>
        <address><email>sdorner@qualcomm.com</email></address>
      </author>
      <author initials="K." surname="Moore" fullname="Keith Moore">
        <organization>Department of Computer Science</organization>
        <address><email>moore@cs.utk.edu</email></address>
      </author>
      <date year="1997" month="August"/>
    </front>
    <seriesInfo name="RFC" value="2183"/>
  </reference>

  <reference anchor="RFC2231">
    <front>
      <title abbrev="MIME Value and Encoded Word Extensions">MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations</title>
      <author initials="N." surname="Freed" fullname="Ned Freed">
        <organization abbrev="Innosoft">Innosoft International, Inc.</organization>
        <address><email>ned.freed@innosoft.com</email></address>
      </author>
      <author initials="K." surname="Moore" fullname="Keith Moore">
        <organization>University of Tennessee</organization>
        <address><email>moore@cs.utk.edu</email></address>
      </author>
      <date year="1997" month="November"/>
    </front>
    <seriesInfo name="RFC" value="2231"/>
  </reference>

  <reference anchor="RFC3629">
    <front>
      <title>UTF-8, a transformation format of ISO 10646</title>
      <author initials="F." surname="Yergeau" fullname="F. Yergeau">
        <organization>Alis Technologies</organization>
        <address><email>fyergeau@alis.com</email></address>
      </author>
      <date month="November" year="2003"/>
    </front>
    <seriesInfo name="RFC" value="3629"/>
    <seriesInfo name="STD" value="63"/>
  </reference>

  <reference anchor="RFC3864">
    <front>
      <title>Registration Procedures for Message Header Fields</title>
      <author initials="G." surname="Klyne" fullname="G. Klyne">
        <organization>Nine by Nine</organization>
        <address><email>GK-IETF@ninebynine.org</email></address>
      </author>
      <author initials="M." surname="Nottingham" fullname="M. Nottingham">
        <organization>BEA Systems</organization>
        <address><email>mnot@pobox.com</email></address>
      </author>
      <author initials="J." surname="Mogul" fullname="J. Mogul">
        <organization>HP Labs</organization>
        <address><email>JeffMogul@acm.org</email></address>
      </author>
      <date year="2004" month="September"/>
    </front>
    <seriesInfo name="BCP" value="90"/>
    <seriesInfo name="RFC" value="3864"/>
  </reference>

  <reference anchor="RFC3986">
   <front>
    <title abbrev="URI Generic Syntax">Uniform Resource Identifier (URI): Generic Syntax</title>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address>
         <email>timbl@w3.org</email>
         <uri>http://www.w3.org/People/Berners-Lee/</uri>
      </address>
    </author>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
      <organization abbrev="Day Software">Day Software</organization>
      <address>
        <email>fielding@gbiv.com</email>
        <uri>http://roy.gbiv.com/</uri>
      </address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
      <address>
        <email>LMM@acm.org</email>
        <uri>http://larry.masinter.net/</uri>
      </address>
    </author>
    <date month="January" year="2005"/>
   </front>
   <seriesInfo name="RFC" value="3986"/>
   <seriesInfo name="STD" value="66"/>
  </reference>

</references>

<section title="Changes from the RFC 2616 Definition" anchor="changes.from.rfc2616">
<t>
  Compared to <xref target="RFC2616" x:fmt="of" x:sec="19.5.1"/>, the following
  normative changes reflecting actual implementations have been made:
<list style="symbols">
  <t>
    According to RFC 2616, the disposition type "attachment" only applies to
    content of type "application/octet-stream". This restriction has been 
    removed, because user agents in practice do not check the content type, and
    it also discourages properly declaring the media type.
  </t>

  <t>
    RFC 2616 only allows "quoted-string" for the filename parameter. This 
    would be an exceptional parameter syntax, and also doesn't reflect actual
    use.
  </t>

  <t>
    The definition for the disposition type "inline" (<xref target="RFC2183" x:fmt="," x:sec="2.1"/>)
    has been re-added with a suggestion for its processing.
  </t>
  <t>
    This specification requires support for the extended parameter encoding
    defined in <xref target="RFC5987"/>.
  </t>
</list>
</t>
</section>

<section title="Differences compared to RFC 2183" anchor="diffs.compared.to.rfc2183">
<t>
  <xref target="RFC2183" x:fmt="of" x:sec="2"/> defines several additional
  disposition parameters: "creation-date", "modification-date",
  "quoted-date-time", and "size". These do not appear to be implemented by
  any user agent, thus have been omitted from this specification.
</t>
</section>

<section title="Alternative Approaches to Internationalization" anchor="alternatives">
<t>
  By default, HTTP header field parameters cannot carry characters outside
  the ISO-8859-1 (<xref target="ISO-8859-1"/>) character encoding (see
  <xref target="RFC2616" x:fmt="," x:sec="2.2"/>). For the "filename"
  parameter, this of course is an unacceptable restriction.
</t>
<t>
  Unfortunately, user agent implementers have not managed to come up with
  an interoperable approach, although the IETF Standards Track specifies
  exactly one solution (<xref target="RFC2231"/>, clarified and profiled for
  HTTP in <xref target="RFC5987"/>).
</t>
<t>
  For completeness, the sections below describe the various approaches that
  have been tried, and explains how they are inferior to the RFC 5987
  encoding used in this specification.
</t>

<section title="RFC 2047 Encoding" anchor="alternatives.rfc2047">
<t>
  RFC 2047 defines an encoding mechanism for
  header fields, but this encoding is not supposed to be used for
  header field parameters - see <xref target="RFC2047" x:fmt="of" x:sec="5"/>:  
</t>
<x:blockquote cite="http://tools.ietf.org/html/rfc2047#section-5">
  <t>
    An 'encoded-word' MUST NOT appear within a 'quoted-string'.
  </t>
  <t>
    ...
  </t>
  <t>
    An 'encoded-word' MUST NOT be used in parameter of a MIME Content-Type or Content-Disposition field, or in any structured field body except within a 'comment' or 'phrase'.
  </t>
</x:blockquote>
<t>
  In practice, some user agents implement the encoding, some do not
  (exposing the encoded string to the user), and some get confused by it.
</t>
</section>

<section title="Percent Encoding" anchor="alternatives.percent">
<t>
  Some user agents accept percent encoded (<xref target="RFC3986" x:fmt="," x:sec="2.1"/>)
  sequences of characters encoded using the UTF-8 (<xref target="RFC3629"/>) character encoding.
</t>
<t>
  In practice, this is hard to use because those user agents
  that do not support it will display the escaped character sequence to the user.
</t>
<t>
  Furthermore, the first user agent to implement this did choose the encoding
  based on local settings; thus making it very hard to use in multi-lingual
  environments.
</t>
</section>

<section title="Encoding Sniffing" anchor="alternatives.sniff">
<t>
  Some user agents inspect the value (which defaults to ISO-8859-1) and
  switch to UTF-8 when it seems to be more likely to be the correct
  interpretation.
</t>
<t>
  As with the approaches above, this is not interoperable and furthermore
  risks misinterpreting the actual value.
</t>
</section>

<section title="Implementations" anchor="alternatives.implementations">

<t>
  Unfortunately, as of August 2010, neither the encoding defined in RFCs 2231
  and 5789, nor any of the alternate approaches discussed above was
  implemented interoperably. Thus, this specification recommends the approach
  defined in RFC 5987, which at least has the advantage of actually being 
  specified properly.
</t>
<t>
  The table below shows the implementation support for the various approaches:
  <cref anchor="impls">Discuss: should we mention the implementation status
  of actual UAs in a RFC? Up to the IESG to decide...</cref>  
</t>


<texttable align="left">
  <ttcol>User Agent</ttcol>
  <ttcol>RFC 2231/5987</ttcol>
  <ttcol>RFC 2047</ttcol>
  <ttcol>Percent Encoding</ttcol>
  <ttcol>Encoding Sniffing</ttcol>
  
  <c>Chrome</c>
  <c>no</c>
  <c>yes</c>
  <c>yes</c>
  <c>yes</c>

  <c>Firefox</c>
  <c>yes (*)</c>
  <c>yes</c>
  <c>no</c>
  <c>yes</c>

  <c>Internet Explorer</c>
  <c>no</c>
  <c>no</c>
  <c>yes</c>
  <c>no</c>

  <c>Konqueror</c>
  <c>yes</c>
  <c>no</c>
  <c>no</c>
  <c>no</c>

  <c>Opera</c>
  <c>yes (*)</c>
  <c>no</c>
  <c>no</c>
  <c>no</c>

  <c>Safari</c>
  <c>no</c>
  <c>no</c>
  <c>no</c>
  <c>yes</c>
  
  <postamble>
  (*) Does
  not implement the fallback behavior to "filename" described in 
  <xref target="disposition.parameter.filename"/>.
  
  </postamble>

</texttable>

</section>

</section>


<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
<section title="Since draft-reschke-rfc2183-in-http-00">
<t> 
  Adjust terminology ("header" -&gt; "header field").
  Update rfc2231-in-http reference.
</t>
</section>


<section title="Since draft-reschke-rfc2183-in-http-01">
<t> 
  Update rfc2231-in-http reference. Actually define the "filename"
  parameter. Add internationalization considerations. 
  Add examples using the RFC 5987 encoding.
  Add overview over other approaches, plus a table reporting
  implementation status.
  Add and resolve issue "nodep2183".
  Add issues "asciivsiso",
  "deplboth", "quoted", and "registry".
</t>
</section>

<section title="Since draft-reschke-rfc2183-in-http-02">
<t>
  Add and close issue "docfallback".
  Close issues "asciivsiso", "deplboth", "quoted", and 
  "registry".
</t>
</section>

<section title="Since draft-reschke-rfc2183-in-http-03">
<t>
  Updated to be a Working Draft of the IETF HTTPbis Working Group.
</t>
</section>
</section>


  </back>

</rfc>